The invention relates to internal combustion engine carburettors, more particularly to carburettors which comprise, for feeding the engine at idling speed, a first duct (called hereinafter an idling duct) which is connected to a source of fuel and to atmosphere via respective calibrated orifices, and which has downstream a third calibrated orifice which is adjustable in cross-section, and a second duct (called hereinafter an air duct) which is connected upstream to atmosphere via a passage having means for controlling its section, the two ducts being connected downstream to that zone of the carburettor intake conduit which is disposed downstream of the main throttle member of the intake conduit, so that the underpressure present in that zone when the throttling member is in the idling speed position causes a rich mixture of fuel and air to flow along the idling duct, and air for diluting the mixture before it is admitted to the engine to flow along the air duct.
A carburettor of that type is disclosed in British Pat. No. 1 217 948 assigned to the assignee of the present invention. In that carburettor, the air duct is connected to a point of the idling speed duct located between the air intake of such duct and the passage of adjustable section, so that any variation in the air flow in such air duct is accompanied by a variation in the same direction of the fuel flow rate of the idling duct.
That prior art arrangement is intended to maintain a substantially constant richness of the mixture supplied to the engine at idling speed when the air flow in the air duct is varied by adjusting the section control means. It gives satisfactory results as long as the variation in air flow in the air duct is low enough; on the other hand, the mixture supplied to the engine has a tendency to be too lean for a low air flow and too rich for a high air flow in the air duct when the range of variation is considerable. In the case of engines with a large cylinder volume, the range of variation in the air flow to be provided is relatively low and enables a substantially constant richness to be maintained for all idling speeds, from the fast idling speed to be provided for a brand new engine to the normal idling speed of a run-in engine. On the other hand, for engines having a small cylinder volume (particularly below 1000 cm.sup.3), the air flow during idling is much higher when the engine is brand new than when the engine has been run-in, and there is a modification in richness.